[fractal] fractal-gtk: add helper text to Join room dialog UI

[Daniel Garcia Moreno <danigm src gnome org>]

commit 27a6d4db6fc3424f33d8ee8f90442058b92be0c3
Author: David Heidelberg <david ixit cz>
Date:   Thu Apr 23 17:44:15 2020 +0200

    fractal-gtk: add helper text to Join room dialog UI
    
    Since spec #room:server is not so widely used, I propose to add
    placeholder text, which can help newbies to make less mistakes.
    
    In future, Fractal could check syntax directly and grey-out Join button
    and colorize input field to warn about incorrect URL.
    
    Signed-off-by: David Heidelberg <david ixit cz>

 fractal-gtk/res/ui/join_room.ui | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/fractal-gtk/res/ui/join_room.ui b/fractal-gtk/res/ui/join_room.ui
index 01c6fe57..9d2fdce9 100644
--- a/fractal-gtk/res/ui/join_room.ui
+++ b/fractal-gtk/res/ui/join_room.ui
@@ -64,6 +64,7 @@
                 <property name="visible">True</property>
                 <property name="can_focus">True</property>
                 <property name="hexpand">True</property>
+                <property name="placeholder_text">#example_room:example_server.org</property>
                 <accessibility>
                   <relation type="labelled-by" target="join_room_name_label"/>
                 </accessibility>